A recent redevelopment project on the Brighton seafront faced a familiar challenge: mapping the contact between the superficial Coombe Deposits and the underlying White Chalk Subgroup. Traditional boreholes were providing point data, but the structural engineers needed a continuous profile to assess dissolution features and potential voids beneath the foundation footprint. The site investigation was supplemented with seismic tomography, deploying a 72-channel seismograph with 5-metre geophone spacing along three intersecting lines. The resulting P-wave velocity tomograms revealed a distinct low-velocity zone at 4 metres depth, later confirmed by a test pit inspection as a solution pipe filled with soft clay. With recorded velocities jumping from 900 m/s in the overburden to over 2,400 m/s in the competent chalk, the foundation design was adjusted to bypass the anomalous zone entirely.
Seismic tomography converts a handful of borehole logs into a continuous velocity cross-section, revealing hidden dissolution features in the Brighton chalk that point investigations alone can miss.
